This invention is in the field of contaminated groundwater cleanup systems, in particular, groundwater that is contaminated by radionuclides.
Recently enacted environmental laws, such as Superfund, require that responsible parties who have contaminated groundwater through industrial processes or by other means must remediate the polluted water. This can be accomplished by pumping the groundwater to the surface and treating it, then discharging it back into the ground or releasing it to a surface water body. Methods for treating contaminated water depend upon the contaminant. Hydrocarbons are common contaminants and can be treated by carbon absorption or by air strippingxe2x80x94allowing the hydrocarbon to volatize to the air. Bioremediation is another means of cleaning groundwater. With this method, microorganisms that consume hydrocarbons are introduced into the water, either in situ or after pumping the water into a tank constructed for the purpose.
Part of post-Cold War government activities is environmental restoration of nuclear weapons installations throughout the country. Groundwater at these sites is commonly contaminated with radionuclides. The typical means of treating water contaminated with hydrocarbons does not work for cleaning water contaminated with radionuclides. Carbon will not absorb most radionuclides. Most radionuclides are non-volatile, so they cannot be air-stripped. It would create a risk of human exposure by inhalation to off-gas radionuclides into the air. Bioremediation is an ineffective clean-up strategy. When radionuclides are consumed by microorganisms, they in turn become radioactive, and remain so after death.
Containment of radioactive groundwater has been employed as a remediation strategy. This method emplaces slurry walls and/or grout curtains underground, where they act as dikes to prevent the radioactive groundwater from flowing off the site, potentially into streams and other surface water features that may be used for recreation, drinking water, irrigation water, and subsistence fishing. Slurry walls and grout curtains are usually not effective in completely containing contaminated groundwater. Radioactive contaminants are often heavy and tend to sink into the groundwater, where they can flow under the slurry wall or grout curtain. Also, these and other containment methods are not permanent constructions; whereas, the radioactive elements remain hazardous for their respective half-lives. The half-lives of elements typically found on Department of Energy nuclear weapons manufacturing sites may range from approximately 125 years for cesium isotopes to 2.3 billion years for some uranium isotopes. The slurry walls and grout curtains would require maintenance, patching and rebuilding for an indefinite period of time, which would need to take place in a hazardous environment.
A further problem with containment methods is the contaminated soils generated in their construction. These soils require disposal, transportation and often, treatment, which involves considerable expense. The risk of exposure to workers must be mitigated throughout the construction process and the treatment, removal, and disposal of the contaminated soils, generating even more expense and liability.
An experimental treatment of contaminated groundwater is freezing. Freezing immobilizes the contaminated groundwater in place, but the process is expensive and temporary, as well as experimental.
The U.S. Department of Energy (DOE) is actively seeking groundwater treatment technologies through solicitations, requests for proposals, and funding for innovative research and development of groundwater treatment systems. Presently, the DOE has adopted a position that because groundwater treatment systems for extracting radionuclides from contaminated groundwater currently do not exist, groundwater cleanup on DOE sites is not technically feasible. The DOE has sought special waivers from Superfund cleanup standards. Waivers have not been granted; however, and the DOE continues to seek out and fund the development of groundwater cleanup technologies in order to attain compliance with existing environmental laws.
The present invention applies the molecular sieve properties of zeolite to the restoration of groundwater and surface water resources. In this process, on-site, below-ground zeolite beds are used to filter radionuclides from groundwater that has been pumped to the zeolite bed. The contaminated water trickles through the zeolite bed. In the course of flowing through the zeolite bed, radioactive ions in the water are exchanged for nonradioactive ions, such as sodium and calcium, that occur naturally in the zeolite crystal structure. The water is collected from the base of the zeolite bed, tested to determine if it has been completely stripped of radionuclides, and then discharged to a surface water body or the groundwater. The radioactive material, which originally was mined from minerals in the ground, again becomes locked into the crystal structure of a mineral, and simultaneously returned to the ground. When the zeolite bed is fully charged with radionuclides, the entire bed is buried with earth. The radioactive elements are thereby disposed in a stable, immobile configuration, which would fulfill the requirements of the existing environmental laws.
The invention relates to a system and method for the in situ removal of metal contaminants, such as radionuclides, and various contaminant metals from contaminated wastewater by the use of a zeolite filter bed.
The invention shall be described for the purpose of illustration only in connection with certain illustrated embodiments; however, it is recognized that various modifications, additions, improvements, and changes may be made by persons skilled in the art without departing from spirit and scope of the invention as disclosed and claimed.